In a digitally controlled oscillator (DCO) based RF frequency synthesizer, the frequency oscillation is typically set by switching on an appropriate number of varactors in the oscillator circuit. For frequency range coverage and frequency resolution purposes, the varactors are typically organized in one or more banks. For example, there may be separate banks for acquisition and tracking modes of operation of the synthesizer in addition to very coarse PVT varactor bank. For a desired output frequency, an appropriate command word is generated and applied to the varactor banks. In this example, the DCO command word is divided between acquisition bit and tracking bits.
Modulation of the data is typically achieved using tracking bank. Both the acquisition and tracking range, however, are of a finite width. If the input tuning word exceeds the range of the varactor banks, then the PLL loop within the frequency synthesizer becomes nonlinear and the output becomes corrupt. Thus, it is important that the DCO input tuning word never exceeds the available linear range of the varactor banks.
Drifting of the center frequency caused by temperature, etc. poses a problem if the drift causes the DCO to creep towards the limits of its tracking range. When the desired center frequency hits the limits of the dynamic range, the tracking bits can no longer track those frequencies resulting in loss of lock and modulation distortion. A similar result occurs if the modulation range is larger than the entire tracking range. This may occur in the implementation of wireless standards requiring very wideband modulation, e.g., WCDMA or Wireless LAN, wherein the required modulation range exceeds the tracking range.